Analysis on integrity of cement sheath in the vertical section of wells during hydraulic fracturing

The sustained casing pressure (SCP) problems occur mainly in intermediate casing that might affect the safety and efficiency of shale gas production. Cement sheath is the main barrier against leakage and migration of gas from the pay zone to the shallow zones. Since the production casing is cemented all the way up to the intermediate casing in the vertical section of the well, the in-situ stress can only act on the intermediate casing and not directly on production casing. Based on this situation, an analytical model considering the well completion steps is established to study the integrity of cement sheath as a two-layer casing-cement system. The effect of mechanical and geometrical parameters of casing and cement are further investigated through a sensitivity analysis. The results show that the tensile failure more likely occurs at the inner face of the cement sheath in the intermediate casing. The casing thickness is in favor of decreasing the tensile stress in the cement sheath. But on the contrary, increasing the thickness and elastic modulus of the cement sheath may increase the risk of tensile failure in the cement sheath while reducing the costs. However, an appropriate increase of back pressure in the annulus during cementing is beneficial to reduce the tensile stress in the cement sheath.